1. Field of the Invention
The present invention relates to an acousto-optic filter drive method realizing reduction of a drive apparatus in size for suppressing the time-fluctuations of output characteristics of acousto-optic filters. The invention further relates to an acousto-optic filter applied to the acousto-optic filter drive method, an optical add/drop multiplexer, an optical communication system, and a selective wavelength extension method.
Recently, there has been demanded an optical communication system with a super-long distance and a large capacity so as to construct future multi-media networks. For realizing the large capacity, there has been investigated and developed the wavelength-division multiplexing (WDM) system because it has advantages of utilizing the wide band and the large capacity of optical fibers with efficiency.
Especially in recent years, it has been demanded to realize not only the optical communication system for transmitting/receiving the WDM optical signal between two stations but also an optical communication system having the ADM function, in which an optical signal having a specific wavelength of the WDM optical signal is selectively passed through a repeater station called the xe2x80x9cnodexe2x80x9d disposed midway of the optical transmission line and in which the optical signals at other wavelengths are dropped at that node or another optical signal is added from the node and transmitted to another node. In order to realize the ADM function, there have been extensively investigated the acousto-optic filters (AOTF).
2. Description of the Related Art
The AOTF is an optical part for rotating the polarization state of light to propagate through an optical waveguide by inducing a refractive index change due to the acousto-optic effect in the optical waveguide thereby to separate/select the light of a specific wavelength. One example of the AOTF will be described in the following.
In the AOTF, as shown in FIG. 12, optical waveguides 502 and 503 are formed in a substrate 501 made of a piezoelectric crystal. For example, the optical waveguides are formed in the substrate of lithium niobate (LiNbO3) by the titanium (Ti) diffusion method. As shown in FIG. 12, the optical waveguides 502 and 503 are individually equipped at their input terminals and output terminals with a port Pin and a port Pad, and a port Pth and a port Pdr. The optical waveguides 502 and 503 intersect each other at two portions, which are equipped with polarization beam splitters (PBS) 504 and 509.
Between the intersecting portions, an inter digital transducer (IDT) 506 is formed over the optical waveguides 502 and 503. A surface acoustic wave is generated by applying RF signals generated by a signal source 507, to the IDT 506 to change the refractive indices of the optical waveguides 502 and 503.
An input light 1 to be inputted to the port Pin is a mixture of a TE mode and a TM mode. This input light 1 is separated by the PBS 504 into the TE mode light and the TM mode light, of which the TM mode light propagates through the optical waveguide 502 and the TE mode propagates through the optical waveguide 503.
Now, when the surface acoustic wave is generated by applying the RF signal of a predetermined frequency, the refractive indices of the optical waveguides 502 and 503 change. Of the input light 1, therefore, only the light having a wavelength to interact on the change in the refractive index rotates the polarized light state. The rotation is proportional to the working length for the light in each mode to interact on the change in the refractive index and to the power of the RF signal. The working length is adjusted by the interval between absorbers 505 and 508 for absorbing the surface acoustic wave to be generated over the optical waveguides 502 and 503 across the IDT 506.
By optimizing the working length and the power of the RF signal, therefore, the TM mode light is transformed into the TE mode light in the optical waveguide 502, and the TE mode light is transformed into the TM mode light in the optical waveguide 503.
As a result, the mode-changed light is outputted as a selected light to the port Pdr by the PBS 509, whereas the light left unchanged in the mode is outputted as the transmitted light to the port Pth.
Here, the transmitted light outputted from the port Pth is prepared by eliminating only the light of the wavelength corresponding to the frequency of the RF signal from the input light 1 inputted to the port Pin. It is, therefore, possible to assume that the AOTF has a rejection function (i.e., band eliminating function).
On the other hand, an input light 2 inputted to the port Pad is likewise separated by the PBS 504 into the TE mode light and the TM mode light. Of these, the TM mode light propagates through the optical waveguide 503, and the TE mode light propagates through the optical waveguide 502. Now, when the surface acoustic wave is generated by applying the RF signal of a predetermined frequency, only the light of the predetermined wavelength rotates its polarized light state so that the TE mode light is transformed into the TM mode light in the optical waveguide 502 whereas the TM mode light is transformed into the TE mode light in the optical waveguide 503. As a result, the light changed in the mode is outputted to the port Pth on the transmitted light side by the PBS 509, and the light left unchanged in the mode is outputted to the port Pdr on the selected light side.
Here, the selected light outputted from the port Pdr is made by selecting only light at a wavelength corresponding to the frequency of the RF signal from the input light 1 inputted to the port Pin. The transmitted light outputted from the port Pth is made by eliminating only light at a wavelength corresponding to the frequency of the RF signal from the input light 1 inputted to the port Pin and by adding only light at a wavelength corresponding to the frequency of the RF signals, from the input light 2 inputted to the port Pad, to the eliminated wavelength. Therefore, the AOTF can be though to have the optical adding/dropping functions.
Moreover, the AOTF is enabled to change the wavelength of the selected/added/transmitted light by changing the frequency of the RF signal so that it functions as a tunable filter.
When lights at a plurality of wavelengths are to be selected/dropped by the AOTF, on the other hand, a plurality of RF signals having different frequencies are applied to the IDT 506 of the AOTF. Therefore, beats are necessarily generated in the surface acoustic waves by the plurality of RF signals so that the center wavelength of the lights to be selected/dropped fluctuates with time in accordance with the beats. As a result, the optical power at the target wavelength to be selected/dropped will fluctuate with time although the power of the input lights and the power of the RF signals are constant.
Simulations have been performed on the case in which lights at four wavelengths are to be selected by two AOTFs, for example.
These two AOTFs are connected in tandem by connecting the portion Pdr of the AOTF at the front step with the port Pin of the AOTF at the back step. The RF signal for selecting a channel 1 and the RF signal for selecting a channel 3 are applied to the AOTF at the front step. The RF signal for selecting a channel 2 and the RF signal for selecting a channel 4 are applied to the AOTF at the back step. With this construction, the simulations have been made by setting the wavelengths of the four waves to be selected to 1545.6 nm, 1547.2 nm, 1548.8 nm and 1550.4 nm and by setting the working length of the AOTFs to 43.1 mm.
The results are illustrated in FIG. 13. In FIG. 13, the ordinate indicates a transmittance at the unit of dB, and the abscissa indicates a wavelength at the unit of nm.
As seen from FIG. 13, the first side lobe on the shorter wavelength side than 1545.6 nm and the first side lobe on the longer wavelength side than 1550.4 nm are at about xe2x88x9210 dB because of the beats. These side lobes cause noises so that the selecting characteristics are the better for the lower side lobes.
In Japanese Unexamined Patent Application Publication No. 10-038908, there is described a drive method for improving the time-fluctuations by driving a plurality of AOTFs connected in tandem such that the phases of the beats of a plurality of RF signals by one AOTF may be different between a plurality of AOTFs.
Here will be described a construction for selecting optical signals of two waves, in which such AOTFs are connected in tandem of two steps.
In FIG. 14, a first AOTF 515-1 and a second AOTF 515-2 of the construction shown in FIG. 12 are connected in tandem by connecting the port Pdr, from which the selected light of the first AOTF 515-1 is outputted, with the port Pin of the second AOTF 515-2.
A RF signal f1 of a frequency f1 to oscillate at a signal source 511-1 is inputted to phase shifters 512-1 and 512-2 for adjusting the phases. The RF signal f1 inputted to the phase shifter 512-1 is adjusted to a phase Ø11 and then inputted to a multiplexer 513-1, and the RF signal f1 inputted to the phase shifter 512-2 is adjusted to a phase Ø12 and then inputted to a multiplexer 513-2.
Likewise, an RF signal f3 of a frequency f3 to oscillate at a signal source 511-3 is inputted to phase shifters 512-5 and 512-6 for adjusting the phases. The RF signal f3 inputted to the phase shifter 512-5 is adjusted to a phase Ø11 and then inputted to the multiplexer 513-1, and the RF signal f3 inputted to the phase shifter 512-6 is adjusted to a phase Ø12 and then inputted to the multiplexer 513-2.
Moreover, an RF signal f2 of a frequency f2 to oscillate at a signal source 511-2 is inputted to phase shifters 512-3 and 512-4 for adjusting the phases. The RF signal f2 inputted to the phase shifter 512-3 is adjusted to a phase Ø21 and then inputted to the multiplexer 513-1, and the RF signal f2 inputted to the phase shifter 512-4 is adjusted to a phase Ø22 and then inputted to the multiplexer 513-2.
Likewise, an RF signal f4 of a frequency f4 to oscillate at a signal source 511-4 is inputted to phase shifters 512-7 and 512-8 for adjusting the phases. The RF signal f4 inputted to the phase shifter 512-7 is adjusted to a phase Ø21 and then inputted to the multiplexer 513-1, and the RF signal f4 inputted to the phase shifter 512-8 is adjusted to a phase Ø22 and then inputted to the multiplexer 513-2.
The RF signals f1 and f2 combined in the multiplexer 513-1, are applied to the IDT in the first AOTF 515-1. The RF signals f1 and f2 combined in the multiplexer 513-2, are applied to the IDT in the second AOTF 515-2.
Here, the phase differences are adjusted to |Ø11xe2x88x92Ø12|=0 degrees and |Ø21xe2x88x92Ø22|=180 degrees by the individual phase shifters 515-1 and 515-2. Then, positions where the selected lights are most attenuated are deviated with time in the individual AOTFs so that the optical power of the selected light 2 can be suppressed in the time-fluctuations when the power of the input light and the power of the RF signals are constant.
Here, the construction of the AOTFs should not be limited to that shown in FIG. 12 but can be modified to that shown in FIG. 15, for example.
In the AOTF, as shown in FIG. 15, optical waveguides 602 and 603 are formed in a piezoelectric crystal substrate 601. These optical waveguides 602 and 603 intersect each other at two portions, at which PBSs 604 and 609 are disposed. Between these two intersecting portions, A SAW guide 310 of a metal film is so formed over the two optical waveguides 602 and 603 as to intersect the two optical waveguides 602 and 603 individually. To the SAW guide 310, there are propagated the surface acoustic waves which are generated by applying the RF signals generated in a signal source 607 to an IDT 606. This IDT 606 is formed over the substrate 601 on the extension in the longitudinal direction of the SAW guide 310. Absorbers 605 and 608 for absorbing the surface acoustic waves are so formed over the substrate 601 as to interpose the SAW guide 310 and the IDT 606.
The operations and effects of this AOTF are similar to those of the AOTF shown in FIG. 12, and their description will be omitted.
Here in the AOTF thus constructed, the number of phase shifters increases according to the number of signal sources and the step number of the tandem connection of the AOTFs. In the AOTFs connected in tandem of three steps in which an input light is a WDM optical signal of thirty two waves so that the thirty two waves can be arbitrarily selected, for example, 32xc3x973=96 phase shifters are required to obstruct the reductions in the size and cost for the apparatus in the entire AOTF.
An object of the invention is to provide an AOTF drive method and an AOTF capable of reduce the number of phase shifters than that of the prior art.
Another object of the invention is to provide a selective wavelength extension method for extending a wavelength to be selected by the AOTF without increasing the number of phase shifters.
Still another object of the invention is to provide an optical add/drop multiplexer where such AOTF is applied.
A further object of the invention is to provide an optical communication system where the optical add/drop multiplexer equipped with the AOTF is applied.
The aforementioned objects are achieved by an AOTF drive method comprising steps of: separating a plurality of RF signals in advance into a plurality of groups; batch-branching the RF signals of each group in accordance with the number of steps of AOTFs; and batch-adjusting the phases of the individual branched RF signals.
The aforementioned objects are also achieved by an AOTF apparatus comprising: a plurality of AOTFS; a plurality of signal generating parts for generating RF signals; a combining part for separating the RF signals generated in the signal generating parts into a plurality of groups and combining the RF signals in each of part; a branching part for respectively supplying outputs of the combining parts to an acousto-optic filters; and a phase adjusting part for adjusting the phases of the outputs of the combining parts so that the phases of beats generated in the plurality of AOTFs are made different.
Moreover, the aforementioned objects are achieved by a selective wavelength extension method for including an RF signal to be extended corresponding to a light to be extended in any of the existing groups even when the number of wavelengths to be selected by the AOTF is extended.
Moreover, the aforementioned objects are achieved by an optical add/drop multiplexer and an optical communication system which comprises such AOTF.
Thus, the number of phase shifters can be substantially reduced than in the case where the RF signals are branched into the number of steps of AOTFs to adjust the phases of the individual branched RF signals. In the AOTF and the optical add/drop multiplexer, therefore, it is possible to simplify the structure of the apparatus and to reduce the number, size, power consumption, and cost of the components.
In the selective wavelength extension method, moreover, the number of wavelengths to be selected by the AOTF can be extended with simplicity and promptness.